This invention relates to a method for producing fine powder from a metal or metal alloy, and more particularly to such a method involving atomization of a stream of molten metal or metal alloy under overpressure by an impinging cone of atomizing gas.
It is known to pass a stream of molten metal through a nozzle and to direct one or more high velocity jets of gas at the emerging stream to break up the stream into small droplets which solidify into particulates of varying sizes. Such gas atomization techniques are valuable for the production of prealloyed (multicomponent) systems as spherical particles which are clean and have low oxygen and nitrogen contents. However, a major disadvantage of most prior art methods is the low yield of fine powders that can be obtained.
There is at present a growing industrial demand for fine and ultrafine metal powders, i.e. powders having a particle diameter smaller than 50 microns and smaller than 10 microns respectively. Presently only about 10 to 20% of the particles of industrially produced powder is within the fine size range, while the ultrafine powder produced is only about 1-3%, making the cost of such powders very high. Accordingly, there is a need to develop gas atomization techniques which can increase the yield of such fine and ultrafine powders.
The diameter of the particles is influenced by the surface tension of the melt from which the powder is produced. For melts of high surface tension, for example copper, copper alloys, and iron alloys, production of fine powder is more difficult and consumes more gas and more energy.
Attempts have been made to improve the yields by altering the surface tension characteristics of various melts using high amounts of oxygen. This approach, however, is not applicable to high surface tension alloys or to materials in which oxygen contamination cannot be tolerated.
Methods for the production of fine powder find particular usefulness in the field of rapid solidification materials. It is known that the rate of solidification of a molten particle of relatively small size in a convective environment such as a flowing gas is roughly proportional to the inverse of the diameter of the particle squared. Accordingly, if the average size of the diameter of the particles of the composition is reduced then the rate of cooling is increased dramatically. This property becomes particularly important in the production of amorphous metals and metal alloys. By producing metal powders having a high percentage of fine and/or ultrafine powders, novel amorphous and related properties may be achieved. Also, novel properties may be achieved in the production of superalloys.
Further, the achievement of smaller particle size can have advantages in the consolidation of materials by conventional powder metallurgy, resulting in a higher packing density, a higher sintering rate, reduced flaw sizes, good rheology, and improved microstructure.
Recently, much experimentation has been performed to improve the atomization process. For example, gas nozzles have been developed for use in confined arrangements, i.e. with the gas outlets in close proximity to the melt outlet, which use an ultrasonic, pulsed gas flow to atomize the melt. Other researchers have stated that high pressure gas flow directed in such a way as to produce aspiration or low pressure conditions at the melt outlet increases the yield of fine powders, the percent of fines increasing with increasing aspiration. However, none of these methods have as yet been successfully adapted to consistently and predictably produce a high percentage of fine and/or ultrafine metal or metal alloy powders on a commercial scale.
The aspiration of melt from the melt nozzle is influenced significantly by the design and placement of the outlet tip of the melt nozzle. Such factors as the taper angle of the outside surface of the tip, the tip length extending below the gas nozzle outlets, and the proximity of the gas nozzle outlets to the tapered outer tip surface greatly influence the degree of aspiration achievable at various gas pressures.
Commonly owned, copending U.S. patent application Ser. No. 926,482, filed Nov. 3, 1986 by R. V. Raman, discloses a method for producing ultrafine metal or metal alloy powder by atomizing a melt using a high gas velocity and low mass ratio of melt flow to gas flow. The method optimizes atomization by achieving a low level of melt aspiration without causing backpressure. This method achieves excellent results using a high gas velocity and by control of the metal to gas flow ratio, the impingement angle at which the gas intersects the melt stream, and the relative placement of the gas and melt outlets.
The most effective and economical operating parameters for this process, however, lie within a relatively narrow range of gas pressures which will produce a low level of aspiration without backpressure. This low aspiration keeps the aspirated melt flow at a low level. Further, this process requires a high gas velocity and short distances between the atomizing zone and both the gas outlet and the melt outlet. The operation at low aspiration and even near-backpressure conditions, the high gas velocity, and the close geometric proximity of the atomizing zone to the gas and melt nozzles can lead to problems of melt splashback and equipment damage if the process slips into backpressure conditions, due for example to a change in gas pressure or damage to the melt nozzle tip.
The backpressure described above is the result of opposing streams of atomizing gas which collide in the atomizing zone. A portion of the gas is deflected upward toward the melt outlet creating pressure which opposes the flow of melt. When the pressure created at the melt outlet exceeds the hydraulic pressure of the melt, backpressure and its accompanying problems, as described above, can occur.
It would be advantageous to find a process for producing a high percentage of fine and ultrafine atomized metal and metal alloys, particularly high surface tension and oxygen sensitive materials, which allows a lower and/or broader gas pressure range, is less stringent in its geometric considerations and is less sensitive to backpressure and less susceptible to splashback problems. The present invention provides such a process, as well as apparatus for carrying out the process.